Power semiconductors typically generate significant heat during operation. In order to ensure efficient operation and avoid failure, some form of cooling or heat sinking is usually provided. Heat sinking usually involves mechanically mounting the semiconductor device to an aluminum heat sink. The efficiency of the heat sink is generally dependent on the pressure exerted on the semiconductor device by the mounting hardware. A heat transfer aid, such as thermal grease (e.g., ceramic-loaded grease), is often used to displace the air between the component and the heat sink with a more thermally-conductive material (the grease), but the application of the grease can be messy. Silicone-based grease can be used, however silicone migration can occur in the electrical circuit.
High voltage and high power semiconductor devices can require electrical isolation from the heat sink to prevent shorting. These devices typically require sufficient thermal coupling to the heat sink to allow proper operation. A thin conformable electrically-insulating pad or layer is often sandwiched between the device and the heat sink during the mounting procedure. Such a mounting procedure typically is done manually and is quite time consuming. The manual mounting of the layer generally prevents rapid, automated production.
A nut and bolt attachment is commonly used to secure the device to the heat sink, however, there are many problems with this type of attachment. If the torque used to fasten the device to the heat sink is too small, the thermal transfer capability is degraded. If the torque is too great, however, the semiconductor can lift the device up from the heat sink at the end of the device opposite the nut and bolt and the thermal transfer capability is degraded.
Other problems include the inconsistency in thermal transfer rate between similarly mounted devices due to variability in the integrity of the individual mounting as well as variations in the materials used between the heat sink and the semiconductor device. In addition, a variety of small parts must be maintained in inventory. Also, these mechanical mounting techniques usually require significant manual labor. A hole must be pre-drilled into the heat sink to accept the bolt. Moreover, the nut and/or bolt can protrude to create packaging design issues.